In recent years, steel sheets used as structural members for electric equipment have been required to have magnetic properties, i.e. high magnetic permeability and magnetic flux density, in addition to mechanical properties for the purpose of enhancing performance of the electric equipment. In particular, rim materials for generators, such as, steel sheets for rotary machine rims of, for example, rotors of large generators for hydraulic power generation and the like are required to have high strength and high magnetic flux densities because large centrifugal forces are applied. Furthermore, in many cases, rim materials for generators are used after being provided with a very large number of punched holes, so that high punchability is required frequently.
Among the above-described magnetic properties, the magnetic permeability increases as the amount of coarse carbides in the steel is reduced and the magnetic flux density increases as the amount of nonmagnetic elements in the steel is reduced. Ultra low carbon steels have been previously used for steel sheets having excellent magnetic properties. However, they cannot achieve high strength although there are growing needs.
Patent Literature 1 discloses a method for manufacturing a high strength hot rolled steel sheet having a high magnetic flux density, wherein Ti and B are added to a Si—Mn steel. In this technology, B is added to improve the hardenability. However, B segregates at grain boundaries, suppresses ferrite transformation and, in addition, induces bainite transformation at lower temperatures, so that a lower bainite microstructure, in which carbides are dispersed in a bainite-lath, is formed easily. Carbides serve as starting points of fine cracks in punching. Therefore, if carbides are dispersed in the bainite-lath, a crack generated at a carbide in the bainite-lath proceeds through the bainite-lath to the boundary of the bainite-lath. Furthermore, this crack proceeds cross the bainite-lath boundary so as to become a macro crack easily. Consequently, cracks are generated easily in a punched surface. Therefore, the punchability, which is the aim of disclosed embodiments, is not achieved.
Patent Literature 2 discloses a hot rolled steel sheet which has a tensile strength of 590 MPa or more and in which less than 10 nm carbides are dispersed in 95% or more of ferrite microstructure on a volume fraction basis and a method for manufacturing the same. In the case of the ferrite microstructure, the material exhibits high local ductility and poor punchability.